1. Field of the Invention
This invention relates to the general field of optical communications and, in particular, to apparatus providing the functionality of two optical devices, such as interleavers or etalons, from the core components of a single device coupled to a four-fiber collimator.
2. Description of the Prior Art
In optical communications, one fiber can carry many communication channels where each channel has its own carrier frequency. The light of different frequencies is merged into the fiber through a device called multiplexer (“mux”) in the art and is later separated into different ports through a device called de-multiplexer (“de-mux”). Mux and de-mux devices typically utilize technologies such as thin-film filters (TFF), array wave-guide gratings (AWG), and optical interleavers.
In dense wavelength division multiplexing (DWDM) optical communication, various frequencies (1/λ) of laser light (channels) are used as carrier signals and are coupled into the same optical fiber, which acts as a waveguide. Data signals are superimposed over the carrier signals and are transported in the waveguide. Since the total usable wavelength range is limited (about a few tens of nanometers), as channel spacing is decreased, more channels can fit into the same optical fiber and greater communication capacity is achieved. Therefore, the ability to operate at ever reduced channel spacing is an important objective in the art.
However, channel spacing is limited by the capability of the multiplexer and the de-multiplexer to combine and separate channels without signal overlap. Currently, the standard for channel spacing is 100 GHz (0.8 nm) and manufacturing costs increase dramatically to implement a channel spacing smaller than 100 GHz.
When the total number of channels is less than about 20, the technology based on thin-film filtering is preferred because of its wide bandwidth, its good thermal stability, and the facility with which channels may be added to the system. When the number of channels is materially higher (e.g., more than about 40), it has been preferable in the art to use optical devices that provide a more uniform loss throughout the channels and exhibit a smaller chromatic dispersion than thin-film technology. For example, devices based on array waveguide gratings (AWG) and diffraction gratings provide these advantages. However, such devices tend to produce a narrower bandwidth than thin-film technology, which severely limits their application. In turn, a cost-effective method for increasing the bandwidth of multiplexing and de-multiplexing devices with uniform insertion loss throughout the channels and minimal chromatic dispersion has been achieved through the use of optical interleavers.
With an interleaver, it is possible to use lower resolution filters to mux/de-mux channels with a channel spacing that is smaller than the filter's frequency resolution. For instance, in order to de-mux eighty channels with a channel spacing of 50 GHz, the interleaver first separates the light into an odd stream and an even stream. The odd stream contains odd-number channels and the even stream contains even-number channels. By doing so, the channel spacing in each stream becomes 100 GHz. Therefore, one can use 100 GHz filters to separate the channels in each stream. Otherwise, one would have to use 50 GHz filters, which are more expensive than 100-GHz ones, to de-mux all 80-channel optical signals.
A conventional free-space de-mux interleaver is a 3-port device. As shown schematically in FIG. 1 in a Michelson interferometer configuration, an optical de-mux interleaver 10 includes a 50/50 beamsplitter 12 combined with a mirror 14 and an etalon structure 16. A single incoming light beam I is incident on a common (input) port 18 and two output beams R,T exit from respective output ports 20,22. A portion of the incoming beam I is first reflected at point 24 of the beamsplitter, and then it is reflected by the mirror 14 and returned to the beamsplitter at point 26, where it is reflected again and transmitted on a 50/50 energy split. The beam returned to point 26 has a phase that is a linear function of its optical frequency. The other portion of the incoming beam I at point 24 of the beamsplitter is transmitted to and phase shifted by the phase optics 16; then it is returned to the beamsplitter at point 26, where itself is also reflected and transmitted on a 50/50 energy split. This beam returned to point 26 has a phase that is a nonlinear function of its optical frequency. The phase difference between the linear phase produced by mirror 14 and the nonlinear phase produced by the phase optics 16 determines which optical frequencies (wavelengths) are in the passband and in the stopband at each of the output ports 20 (the reflection beam R) and 22 (the transmission beam T). The etalon 16 typically includes a tuner and consists preferably of a Gires-Tournois etalon.
Dual-fiber collimators are widely used in DWDM, such as in fixed wavelength filters, optical switches, and interleavers, due to their compactness and reliability. In a dual-fiber collimator, two bare fibers are placed next to each other inside a capillary. Therefore, the center-to-center distance between two fibers, for standard SMF-28 fibers, is nominally 125 um. Both fibers share one collimating lens and the tips of the fibers are located near the focus plane of the collimating lens. As a result, the collimated beams from the two fibers lie at a small angle determined by the center-to-center distance and the focal length of the collimating lens. For instance, in a dual-fiber collimator with a 6.5-mm focal length, the angle between the two collimated beams is about 1.1 degrees.
Multiple fibers can similarly share a collimating lens. Accordingly, due to their compactness, in recent years a variety of multiple-fiber collimators have been used, mainly for optical switches. In such optical-switch applications, the switch works for all wavelengths in a band, there is no ITU alignment requirement, and the insertion loss (the most important parameter in most applications) is low, which renders the use of multiple-fiver collimators very desirable.
As well understood in the art, the free spectral range (FSR) of an etalon is determined by the cavity length; that is, the distance between the two reflective surfaces of the etalon. When the incident beam is not normal to the cavity (such as caused by the position of the input fiber), the effective cavity length is reduced according to a cosine law. As a result, the FSR is increased and the transmission peaks of the cavity are shifted. For instance, using a dual-fiber collimator with a 6.5-mm focal length for a 50 GHz etalon, the effective cavity length is reduced by 0.14 um. (For the purposes of this description, um≡μm.) It is well known that, when the cavity length is changed by a distance equal to one-half wavelength, the transmission peaks will shift by one FSR. Therefore, a 0.14 um change in the effective cavity length will cause the transmission peaks to shift by 8.9 GHz.
Furthermore, due to the tolerance of both the capillary and the fiber diameters, the center-to-center capillary distance could cause an error of several um, as illustrated in ideal and more realistic configurations in FIGS. 2 and 3, respectively, and the focal length of the collimating lens could have a tolerance of a few percent. Thus, the focal-length error and the fiber-distance error can cause the angle between the two collimated beams to be materially off the desired design value. Therefore, it is important to control these parameters to obtain the desired output.
In dual-fiber interleaver applications, such as described in U.S. Pat. No. 6,587,204, the input collimator has two fibers. As illustrated in FIG. 4, one fiber FI is for input, the other fiber FB for the reflection channel. The transmission channel only needs one fiber FT to receive the transmission beam. The core optics (beam splitter 12, mirror 14, etalon 16 with tuner) of the interleaver is placed inside a housing (not shown) and the collimators 30 and 32 are aligned to optimize the performance. The interleaver is then tuned conventionally to align its output reflection and transmission peaks to a predetermined grid, for instance ITU. (See U.S. Pat. No. 6,816,315 for an example of suitable tuning apparatus.) All the even channels (wavelengths) of the ITU grid are directed to the transmission port and all odd channels (wavelengths) to the reflection port, or vice versa. The present invention is directed at providing the functionality of two dual-fiber interleavers with the core optics of a single one.